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Optical amplifiers that employ a doped medium that are excited by a pump source are well known. The pump source excites at least a portion of the dopant atoms to create a population inversion of electrons. This population inversion of electrons provides stimulated emission when an incoming signal photon strikes one or more of the excited electrons. These pumped optical amplifiers are typically constructed out of one or more optical fibers that are doped with an optically active dopant. If more than one fiber is used, for example in imaging applications, the optical fibers are typically rigidly bundled together at the input and output of the optical fiber to preserve the spatial orientation of the incoming photons by preventing an optical fiber from shifting in position. Although doped optical fibers can provide high amplification of an optical sign with a low noise figure, there are problems associated with doped optical fiber amplifiers.
For instance, when bundled together, a plurality of doped optical fiber amplifiers can be difficult to construct so as to maintain the optical fibers in the desired orientation. In addition, it is important to maintain good optical isolation between the optical fibers in a compact package. Furthermore, doped optical fiber amplifiers require coupling to and from the optical fiber it is connected to. This coupling can cause signal loss due to the losses inherent in the use of connectors and due to the alignment errors between the connectors themselves and the fiber optic cable.
It would therefore be advantageous to provide an optical amplifier that provides amplification of an optical signal that provides both high gain and a low noise figure without the inherent complexity of forming bundles of optical fibers with the attendant alignment and connector losses.
An active optical amplifier is disclosed in which a unitary optical amplifier, is constructed from a unitary optically transparent chip that has been doped so as to be optically active. The chip amplifies incoming signal photons when excited by a pump source of sufficient energy. The unitary optical amplifier receives input photons and pump laser energy and provides output photons that have the same spatial orientation, phase, and polarization as the corresponding input photons. The unitary optical amplifier may be constructed from glass such as silicate and phosphate glass or other materials that may be doped so as to become optically active. Various materials may be selected based on their electron structure to act as dopants. In one embodiment Erbium is used as a dopant. Alternatively, Neodymium may be used. In addition, an optically transparent heat transfer medium may be thermally coupled to the unitary optical amplifier in order to transfer heat away from the amplifier. In one embodiment, the optically transparent heat transfer medium is a thin diamond plate that is thermally coupled to a surface of the unitary optical amplifier. A heat sink or electro-thermal system may be employed to transfer heat from the optically transparent heat transfer medium.
A laser direction and ranging (LADAR) may be constructed from the active optical amplifier by further including first imaging optics to focus the input photons onto the surface of the unitary optical amplifier and second imaging optics to focus the output photons from the active unitary optical amplifier onto a focal plane image sensor array. The electronic signals from the focal plane image sensor array may then be displayed on a conventional display. A calibration system may be employed to provide offset correction values for each pixel to account for variations in the gain of the unitary optical amplifier. Alternatively, an optically transparent heat transfer medium, such as a diamond, may be thermally coupled to the active optical amplifier to remove heat therefrom.
The active optical amplifier may include a plurality of unitary optical amplifier chips configured in a stacked arrangement. Each of the amplifier chips receives photons from the preceding adjacent amplifier chip and provides output photons to the next adjacent amplifier chip. At least one optically transparent heat transfer medium, such as a diamond plate, may be thermally coupled to the juxtaposed faces of adjacent amplifier chips to remove heat therefrom.